A DETERMINATION OF THE THICK DISK CHEMICAL ABUNDANCE DISTRIBUTION IMPLICATIONS FOR GALAXY EVOLUTION

We present a determination of the thick disk iron abundance distribution obtained from an in situ sample of F/G stars. These stars are faint, 15≲V≲18, selected on the basis of color, being a subset of the larger survey of Gilmore and Wyse designed to determine the properties of the stellar populations several kiloparsecs from the Sun. The fields studied in the present paper probe the iron abundance distribution of the stellar populations of the galaxy at 500-3000 pc above the plane, at the solar Galactocentric distance. The derived chemical abundance distributions are consistent with no metallicity gradients in the thick disk over this range of vertical distance, and with an iron abundance distribution for the thick disk that has a peak at -0.7 dex. The lack of a vertical gradient argues against slow, dissipational settling as a mechanism for the formation of the thick disk. The photometric and metallicity data support a turn-off of the thick disk that is comparable in age to the metal-rich globular clusters, or ≳12 Gyr, and are consistent with a spread to older ages.